1. Field of the Invention
The present invention relates to a reference bias generating circuit; and, more particularly, to a low-voltage reference bias generating circuit.
2. Description of Related Art
In general, a circuit of an electronic system is formed in an integrated chip including a plurality of active elements and passive elements. Each of the elements in the electronic system requires a reference bias circuit. The reference bias circuit generates a reference voltage and a reference current for stable operation of the electronic system. Therefore, the bias circuit is an important element in an electronic system.
Overall power consumption of an electronic system has increased due to the diversification of applications and the increments of functions to provide to a user. Accordingly, it is important to improve the battery efficiency of an electronic system that is not continuously applied with a predetermined voltage. In order to reduce the power consumption, circuits with a low supply voltage have to be developed and a reference bias circuit has to be also advanced to operate in a low supply voltage.
In general, elements of an electronic system have properties that change according to a temperature. For example, passive elements such as resistors or inductors have a resistance value increasing in proportion to a temperature. Also, a semiconductor element having particular conjunction (PN junction) such as a diode or a transistor has a resistance value increasing in reverse proportion to a temperature. Such elements may have linear property or non-linear property for the temperature. Accordingly, a reference bias circuit is also generally influenced by the temperature. In a system with various ICs, the increment of an internal temperature or an external temperature influences badly the performance of an electronic system. Therefore, there has been a demand for a bias circuit that can be driven with a low supply voltage and less sensitive to a temperature variation.
Hereinafter, a bandgap bias circuit for generating uniform bias currents/voltages regardless of temperature variation will be described.
FIG. 1 is a conventional bandgap bias circuit for generating a uniform bias voltage.
Referring to FIG. 1, the conventional bandgap bias circuit includes first to third transistors MM1, MM2, and MM3, first to third bipolar junction transistors Q1, Q2, and Q3, and an OP-AMP.
Here, a voltage ΔVBE applied to a first resistor RR1 is a difference between a base-emitter voltage VBE1 of the first bipolar junction transistor Q1 and a base-emitter voltage VBE2 of the second bipolar junction transistor Q2.
A current I3 flowing to the third transistor MM3 by mirroring the RR1 current is proportional to the current flowing through the first resistor RR1. Therefore, a reference voltage Vref outputted from the bandgap bias circuit is the sum of a voltage V1 applied to both terminals of a second resistor RR2 and a base-emitter voltage VBE3 between the emitter and base of the third bipolar junction transistor Q3.
Here, the voltage V1 applied to the both terminals of the second resistor RR2 can be expressed as Eq. 1 based on the Ohm's law.V1=I3×RR2  Eq. 1
The voltage VBE3 between the emitter and the base of the third bipolar junction transistor Q3 is referred to as ‘V2’. As described above, each of the elements has its property changing according to temperature. Therefore, the reference voltage Vref according to the temperature can be expressed as Eq. 2.Vref32 α1V1+α2V2  Eq. 2
In Eq. 2, α1 denotes a temperature coefficient for a resistance value of the second resistor RR2, and α2 denotes a temperature coefficient about VBE3 of the third bipolar junction transistor Q3.
In order to satisfy the reference voltage having a constant value according to the temperature, the differentiation of Eq. 2 for temperature must have relation of Eq. 3.
                                                                        α                1                            ⁢              Δ              ⁢                                                          ⁢                              V                1                                                    Δ              ⁢                                                          ⁢              T                                +                                                    α                2                            ⁢              Δ              ⁢                                                          ⁢                              V                2                                                    Δ              ⁢                                                          ⁢              T                                      =        0                            Eq        .                                  ⁢        3            
The sum of two differential values in Eq. 3 will be 0 if two values are the same with the opposite sign.
In a conventional bandgap bias circuit, a diode-PN junction voltage VBE of a bipolar junction transistor has negative relation in proportion to temperature variation. A base-emitter voltage difference of two bipolar junction transistors having a different current amount has positive relation in proportion to temperature variation due to a difference of voltage gradients. Therefore, a reference voltage Vref generated from a bandgap bias circuit can be expressed as Eq. 4.
                              V          ref                ≈                              V                          BE              ⁢                                                          ⁢              3                                +                                                    R                2                                            R                1                                      ⁢            Δ            ⁢                                                  ⁢                          V                              BE                ⁢                                                                                                      ≈                              V                          BE              ⁢                                                          ⁢              3                                +                                    k              ·                              V                t                                      ⁢            ln            ⁢                                                  ⁢            n                          ≈                  1.25          ⁢                                          ⁢          V                                    Eq        .                                  ⁢        4            
The reference voltage Vref generated from this bandgap bias circuit is decided based on the sum of the base-emitter voltage difference ΔVBE and the base-emitter voltage VBE3 of the third bipolar junction transistor Q3. Here, it is possible to provide a low reference voltage less sensitive to temperature variation by attenuating a temperature variable which can be controlled a resistance ratio RR1/RR2 and a coefficient k having a temperature characteristic gradient opposite to the base-emitter voltage VBE3 of the third bipolar junction transistor Q3.
However, such a bandgap bias circuit according to the prior art has an excellent temperature compensation characteristic at around a reference voltage of 1.25V theoretically as shown in Eq. 4. Therefore, it cannot be applied to circuits using a supply voltage lower than 1.2V. So, there is a demand for developing an apparatus and method for stably and uniformly providing a reference current and a reference voltage even in a low supply voltage such as lower than 1.2V.